ONIOM principle

A variety of QM/MM schemes have been presented over the years. At a conceptual level the methods are very similar, although the implementations can be very different. The ONIOM hybrid method that we discuss in this contribution, however, is conceptually quite different from other QM/MM schemes [7-10], QM/MM methods can combine only a QM method with an MM method, while ONIOM can combine QM methods with QM methods as well as QM methods with MM methods. Also, ONIOM can in principle combine any number of methods, although our current implementation is limited to three [11]. 

The main difference between the current implementations of IMOMM, IM-OMO, and ONIOM and the majority of other available QM/MM methods is related to the handling of the interaction between the QM and the MM regions. In principle, in any hybrid QM/MM method the total energy of the whole system can in all generality be expressed as ... 

If computational power permits, first-principles molecular modeling such as ab initio MD can provide the most accurate information without adjustable parameters. However, even the most powerful computer system available today is limited to first-principles solution of molecular information with a certain size. As a result, the hybrid methods such as QM/MM or ONIOM (where different levels of theory for one calculation are involved) are often used (Fig. 1). As an example of future prospects, molecular design and characterization of nanocomposites using computer simulation are also briefly mentioned. Atomistic modeling of clay nanocomposites is a very promising field in clay mineral materials science. 

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